Since the advent of scanning tunneling microscopy there are a number of techniques such as atomic force microscopy, magnetic force microscopy, and scanning piezo-response microscopy that have emerged for sensitive surface characterization of samples. Magnetic resonance force microscopy is a scanned probe instrument which combines the three-dimensional imaging capabilities of magnetic-resonance imaging with the high sensitivity and resolution of atomic-force microscopy. Magnetic resonance force microscopy enables nondestructive, chemical-specific, high resolution microscopic studies and imaging of subsurface properties of a range of materials.
Magnetic resonance force microscopy incorporates the principles of scanning tunneling microscopy and magnetic resonance-type detections such as electron spin resonance and nuclear magnetic resonance. Magnetic resonance force microscopy is used to detect unpaired electron spins in semiconductor samples known as E′ centers and magnetic moments in magnetic samples.
The basic elements of magnetic resonance force microscopy include a mass-loaded cantilever with an attached magnetic tip to sense the force from electron spins or magnetic moments of the sample. An alternating current oscillating microwave magnetic field, in combination with the magnetic field from the magnetic tip, sets up a “resonance slice” in the sample, where the condition of the electron spin resonance is satisfied. The slice is a bowl-shaped surface that extends roughly 250 nm below the magnetic tip and into the sample.
The cantilever with the magnetic tip vibrates in the plane parallel to the sample surface. The frequency of the vibration is picked up by a laser beam shined on the cantilever and through an interferometer. The vibration of the cantilever tip causes the resonant slice to swing back and forth in the sample. When the resonant slice swings through the location of an electron spin, the spin will be cyclically inverted, resulting in a small cantilever frequency shift.
Current magnetic resonance force microscopy utilizes an oscillating microwave magnetic field to excite electron spin resonance. A microwave field of at least 5 Oe oscillating at 3 Ghz is usually required to drive electron spin resonance into the Ghz regime. This set-up is bulky and measurements are usually limited to very low temperatures.